1. Field of the Invention
The present invention relates to a scroll compressor for compressing a coolant or an air, particularly to a scroll compressor having a orbiting scroll and an Oldham""s coupling comprising an Oldham""s ring for providing a swing or orbital movement to the orbiting scroll, with the sliding parts of the Oldham""s coupling having an improved wear resistance.
2. Description of the Related Art
Since a scroll compressor has a higher efficiency, a higher reliability and a lower operation noise than compressors of other types, it has been widely used in various fields in industry, such as in a freezer or in an air conditioner.
Such a scroll compressor includes a fixed scroll fixed in the compressor""s frame structure, and an orbiting scroll located in a position opposite to the fixed scroll. The fixed scroll has a circular base plate providing a spiral wrap structure formed thereon. Similarly, the orbiting scroll also has a circular base plate providing a spiral wrap structure formed thereon. The fixed scroll and the orbiting scroll are arranged in a manner such that both spiral wrap structures are mutually engaged with each other. In this way, when the orbiting scroll is caused to perform an orbiting movement with respect to the fixed scroll, a fluid like a gas to be compressed can be continuously compressed so as to be discharged.
In general, a scroll compressor is formed such that in order to enable the orbiting scroll to have a orbital movement, a motor is connected with the back surface of the base plate of the orbiting scroll via a crank shaft, and an Oldham""s ring of an Oldham""s coupling is provided between the back surface of the base plate of the orbiting scroll on one hand and the said frame structure on the other. In fact, such an Oldham""s ring is formed by a ring-like member. On either surface of the Oldham""s ring there are formed two projections (Oldham""s keys) in the diameter direction of the Oldham""s ring. However, the two projections formed on one surface of the Oldham""s ring and another two projections formed on the other surface of the Oldham""s ring in such a manner that every two adjacent projections are separated from each other by an angle of 90 degrees in the circumferential direction of the Oldham""s ring.
On the other hand, key grooves are formed on the frame structure of the scroll compressor and on the back surface of the base plate of the orbital scroll. The two projections (Oldham""s keys) formed on one surface of the Oldham""s key are freely slidably engaged in the grooves formed on the back surface of the base plate of the orbiting scroll, while another two projections formed on the other surface of the Oldham""s ring are freely and slidably engaged in the grooves formed on the frame structure. In this way, when the crank shaft is rotatably driven by the motor, the respective projections of the Oldham""s ring of the Oldham""s coupling will be forced to move reciprocatingly in and along the grooves formed on the frame structure and the grooves formed on the back surface of the base plate, thereby rendering the orbiting scroll to have a orbital movement.
However, since the respective projections of the Oldham""s ring of the Oldham""s coupling are caused to move reciprocatingly in and along the grooves formed on the frame structure and the grooves formed on the back surface of the base plate, these projections are easy to wear away. In particular, when a load (key load) acting on the projections are large, the surface pressure acting on the sliding surface of the projections of the Oldham""s ring (sliding in and along the grooves formed on the frame structure and the grooves formed on the back surface of the base plate) will be increased. As a result, it will be difficult for an lubricant oil to form a continuous layer or film on the sliding surface of each projection, resulting in a problem that the projections will be worn away easier.
Furthermore, in recent years, with the use of an inverter-controlled speed-variable scroll compressor, an ON/OFF frequency is reduced and the discharge amount of compressor is continuously controlled according to an actual load. In this way, an attempt has been made trying to save energy, and under such a situation, since a low speed rotation of a scroll compressor will cause the projections to slide at a low speed, it is difficult to form an oil film or a continuous oil layer on the sliding surface of each projection, resulting in a problem that the projections will be worn away too soon.
Further, in the case where HFC coolant that does not contain chlorine molecules is used, the sliding surface of each projection will have a low lubricity. As a result, under a condition where key load has become large and sliding speed has become low, an abrasion amount on each projection will be undesirably increased.
In view of the above, there has been suggested that when an Oldham""s ring of an Oldham""s coupling is made of an aluminium material, the sliding surface of each projection is subjected to an electric plating treatment called SiC-dispersed plating, so that each projection is allowed to obtain an improved wear resistance (Japanese Unexamined Patent Application Publication No. 3-906383). In addition, there has also been suggested that when an Oldham""s ring of an Oldham""s coupling is made of a sintered iron, the sliding surface of each projection is subjected to a surface treatment to form an iron boride film thereon, so that each projection is allowed to obtain an improved wear resistance (Japanese Unexamined Patent Application Publication No. 6-81779).
However, the above-described conventional treatments have been found to be associated with the following problems. Namely, the electric plating treatment called SiC-dispersed plating makes it necessary to conduct a size management in order to control the thickness of each electric plating layer, but fails to avoid an abrasion on each projection when a plating layer has peeled off from its original position.
In the case where the sliding surface of each projection is subjected to a surface treatment in order to form an iron boride film thereon, although the hardness of each projection can be increased, there will be an abrasion on each counterpart member on which a corresponding projection slides, i.e., there will be an abrasion on the key grooves formed on the frame structure as well as on the back surface of the base plate of the orbiting scroll.
On the other hand, although it is allowed to consider enlarging the sliding area on each projection so as to increase its wear resistance, however, enlarging sliding area will require the Oldham""s coupling to be made in a large size. As a result, each conventional scroll compressor has to be modified significantly in its structure, which is however practically impossible.
Accordingly, it is an object of the present invention to provide an improved scroll compressor by increasing the wear resistance of its Oldham""s keys of the Oldham""s coupling and its counterpart members, that is so called Oldham""s key grooves, sliding therewith.
In order to arrive at the above object, there is provided a scroll compressor comprising a fixed scroll having a base plate and a spiral wrap structure formed on the base plate; a orbiting scroll having a base plate and a spiral wrap structure formed on the base plate, said orbiting scroll being set in a manner such that the spiral wrap structure of the orbiting scroll is engaged with the spiral wrap structure of the fixed scroll; an Oldham""s ring provided between the back surface of the base plate of the orbiting scroll and a frame structure of the scroll compressor; and a driving means connected to the back surface of the base plate of the orbiting scroll through a crank shaft, capable of cooperating with the Oldham""s ring to cause the orbiting scroll to have a orbital movement. In particular, the Oldham""s ring is so formed that at least its sliding surfaces allowing the back surface of the base plate of the orbiting scroll as well as the frame structure to slide thereon, are formed by a base material and silicon contained in the base material, the surfaces of silicon remaining on said sliding surfaces are formed into flat surfaces, a ratio of silicon portions formed into flat surfaces to an entire sliding area of the Oldham""s ring is in a range of 3% to 20%, preferably 5% to 15%.
With the use of the above construction, it is possible to improve the wear resistance of the sliding surfaces of the Oldham""s ring of the Oldham""s coupling. Further, in the case where the surfaces of silicon remaining on the sliding surfaces of the Oldham""s ring are formed into flat surfaces, since the base material will be slightly cut away in a comparison with the silicon, an oil film or layer may be formed in each of the recess portions between each particle of the silicon formed by such cutting. In this way, it is allowed to properly maintain a lubricating oil in these recess portions, thus effectively inhibiting an abrasion on the back surface of the base plate of the orbiting scroll (serving as a counterpart member for the Oldham""s ring of the Oldham""s coupling) as well as an abrasion on the frame structure of the scroll compressor (also serving as a counterpart member for the Oldham""s ring of the Oldham""s coupling). At this time, if the base material contains too much silicon, the area of the recess portions (formed by cutting) of the base material will become smaller, rendering it difficult to keep the lubricating oil on these recess portions. On the other hand, if the base material contains too little silicon, it will be impossible to maintain a desired wear resistance on the sliding surface of the base material. For this reason, in the present invention, a ratio of silicon portions formed into flat surfaces to an entire sliding area of the base material is in a range of 3% to 20%, preferably 5% to 15%.
Further, according to the present invention, the silicon is an initial crystal silicon in the form of silicon particles having a size of 100 xcexcm or smaller, preferably 50 xcexcm or smaller. If the particle size is too large, the area of the recess portions (formed by cutting) will become small, rendering it difficult to maintain a lubricating oil in these recess portions. For this reason, silicon particles are formed into a size of 100 xcexcm or smaller, preferably 50 xcexcm or smaller. Further, the initial crystal silicon has a high hardness as well as an excellent wear resistance.
The base material forming the Oldham""s ring of the Oldham""s coupling may be a metal containing aluminium.
Moreover, the Oldham""s ring of the Oldham""s coupling may be formed by forging or die-casting. In this way, it is possible to manufacture the Oldham""s ring at a reduced cost and with an improved productivity.